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Global Positioning System (GPS)

What is GPS?

GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere.

The three parts of GPS are:•Space Segment•User segment•Ground Control segment

What is the GPS?Orbiting navigational satellites

Transmit position and time dataHandheld receivers calculate

latitudelongitudealtitudevelocity

Developed by Department of Defense

History of the GPS1969—Defense Navigation Satellite System

(DNSS) formed1973—NAVSTAR Global Positioning System

developed1978—first 4 satellites

launchedDelta rocket launch

History of the GPS1993—24th satellite

launched; initial operational capability

1995—full operational capability

May 2000—Military accuracy available to all users

Components of the System

The Space Segment

The Control Segment

The User Segment

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Components of the System

• Space segment

• Control segment

• User segment

Components of the SystemSpace segment24 satellite vehiclesSix orbital planes

Inclined 55o with respect to equator

Orbits separated by 60o

20,200 km elevation above Earth

Orbital period of 11 hr 55 min

Five to eight satellites visible from any point on Earth

Block I Satellite Vehicle

The GPS Constellation

GPS Satellite Vehicle Four atomic clocks Three nickel-cadmium batteries Two solar panels

Battery charging Power generation 1136 watts

S band antenna—satellite control 12 element L band antenna—user

communication

Block IIF satellite vehicle (fourth generation)

GPS Satellite VehicleWeight

2370 poundsHeight

16.25 feetWidth

38.025 feet including wing span

Design life—10 yearsBlock IIR satellite vehicle assembly at Lockheed Martin, Valley Forge, PA

Components of the SystemUser segmentGPS antennas & receiver/processorsPositionVelocityPrecise timingUsed by

AircraftGround vehiclesShipsIndividuals

Components of the System

Ground control segmentMaster control station

Schreiver AFB, ColoradoFive monitor stationsThree ground antennasBackup control system

GPS Communication and Control

GPS Ground Control Stations

Triangulation

Geometric Principle: You can find one location if you know its distance from other, already-known locations.

How does Gps Works?

Triangulation

Triangulation

Signal From One Satellite

The receiver is somewhere on this sphere.

Signals From Two Satellites

Three Satellites (2D Positioning)

Three Dimensional (3D) Positioning

3-D Trilateration1 Satellite 2 Satellites

3 Satellites

Sources of Signal Interference

Earth’s Atmosphere

Solid Structures

Metal Electro-magnetic Fields

Obstruction

Atomic Clocks

GPS satellites use Atomic Clocks for accuracy, but because of the expense, most GPS receivers do not.

Line of Sight Transmissions

Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS transmission are line-of-sight transmissions.

Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.

Light Refraction

Sometimes the GPS signal from the satellite doesn’t follow a straight line. Refraction is the bending of light as it travels through one media to another.

Signal Refraction

Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.

Signal Interference

Sometimes the signals bounce off things before they hit the receivers.

Satellite Distribution

When the satellites are all in the same part of the sky, readings will be less accurate.

PDOP

All of this combines to make the signal less accurate, and gives it what we call a high “PDOP.”

PDOP = Positional Dilution of Precision

11,000 miles

11,000 miles

11,000 miles

11,000 miles

•A PDOP of <4 is excellent•A PDOP of 4-8 is good •A PDOP of >8 is poor

In a Nutshell

Application of GPS Technology

• Location - determining a basic position• Navigation - getting from one location to

another • Tracking - monitoring the movement of people

and things • Mapping - creating maps of the world • Timing - bringing precise timing to the world